The invention relates to an arrangement for ascertaining and reducing temperature-related dimensional discrepancies of measurement systems arranged in parallel, and a method for compensating for the ascertained dimensional discrepancies.
German Patent 195 31 676 C1 discloses an apparatus for compensating for guide tolerances in multi-axis positioners. In this apparatus, a second guide support is mounted, shiftably with respect to a first axis by way of an associated positioner drive, on a first guide support. A further guide support or an object is mounted, shiftably with respect to a second axis by way of an associated positioner drive, on the second guide support. The respective shift on the guide supports is continuously ascertained by way of a coordinate measurement apparatus associated with the supports. For this purpose, respective measurement devices are aligned parallel to the guide on the guide supports, and discrepancy measurement apparatuses that are acted upon by the individual measurement devices are arranged respectively on the guided part. The discrepancy measurement apparatus is arranged in such a way that the discrepancy measurement signal in each case indicates at least one positional discrepancy transversely to the direction of the incident measurement beam.
German Patent 195 31 676 C1 thus does not disclose how temperature-related measurement discrepancies of measurement systems arranged in parallel can be ascertained or compensated for.
Machines that have a gantry structure are especially susceptible to problems because they have two guides, two drive systems, and two measurement systems that are provided for displacement in one axial direction. To ensure that this gantry structure does not jam in the event of a non-homogeneous temperature-related expansion of the measurement systems whose output signals are used to control the drive assemblies, differences in the temperature-related expansion of the two longitudinal measurement systems must be compensated for.
It is known in the existing art to measure the temperature at each of several locations on the arrangement. Taking into account the specific coefficients of thermal expansion, the temperature-related longitudinal changes of the measurement systems are then calculated and compensated for.
It is disadvantageous in this context that the coefficients of thermal expansion of the longitudinal measurement systems and of the machine can be only imprecisely determined. Especially with large machines, the expansion of the machine on a first side is influenced by the expansion, due to the temperature difference, on a second side of the machine. The longitudinal measurement systems mounted on the two machine sides are also shifted relative to one another by the change in the geometry of the machine. Such a shift of the measurement systems inevitably results in jamming of a gantry assembly whose drive systems on each side are controlled on the basis of the output signals of the measurement systems.
An arrangement is described in which a shift of one part of the measurement systems relative to another part, as a result of temperature-dependent geometry changes, can no longer occur. Alternatively, an arrangement is described for ascertaining temperature-related dimensional discrepancies of measurement systems arranged in parallel, and a method for compensating for them.
In one embodiment, the invention is an arrangement for reducing temperature-related dimensional discrepancies of two measurement systems arranged in parallel for machines having a gantry structure, that includes at least one connecting bridge having a substantially temperature-independent geometry, disposed between the two measurement systems, and connecting points at which the at least one bridge meets the two measurement systems providing an immovable connection with the measurement system in a measurement direction. The invention can also include at least one measurement device adapted to measure a shift between the at least one connecting bridge and at least one of the two measurement systems.
In another embodiment, the invention is a method for reducing temperature-related dimensional discrepancies of at least two measurement systems arranged in parallel in a machine having a gantry structure, having the steps of determining a shift between end points of the at least two parallel measurement systems in a measurement direction, creating a model to estimate the shift and correction values, and correcting measured values of the at least two parallel measurement systems using the correction values determined in the model.
The arrangement according to the present invention has the advantage that because of a connecting bridge whose geometry has at most a very small dependence on temperature, either two measurement systems aligned in parallel are immobilized directly with respect to one another, or changes in the measurement systems in the measurement direction are ascertained by way of additional measurement devices attached to the connecting bridge, in the region of the ends of the measurement systems. The values ascertained by the measurement devices on the connecting bridge are used to compensate for the measurement values ascertained by the measurement systems aligned in parallel. The connecting bridge can be implemented either by way of a material with the lowest possible coefficient of thermal expansion, or by way of a light beam that is used to determine the shifting of one of the parallel longitudinal measurement systems relative to the other.